In the conventional "activated sludge" process for the treatment of sewage, an aeration tank of substantial size is followed by a large settling and clarifying tank. The material being treated is aerated in the first tank for a period of time sufficient to achieve, through the action of oxygen and biological life such as aerobic bacteria and protozoa, desired biochemical removal of organic waste. In the settling and clarifying tank, as its name indicates, sludge settles to the lower portion of the tank and a relatively clear effluent rises to the top.
Sewage or other aqueous waste material containing solid particles suspended therein is introduced at the inlet of the aeration tank where, together with activated sludge that is continuously returned to the tank, it forms "mixed liquor." Mixed liquor contains not only aqueous waste material but also an active "biomass" that has been in contact with the aqueous waste material in the presence of oxygen for respiration. After it has remained in the aeration tank for a period of time, mixed liquor is continuously withdrawn from the outlet of the tank and delivered to the settling and clarifying tank.
In the settling and clarifying tank, a substantial portion of the suspended solids settles out as sludge (including as its volatile portion the biomass referred to above), and is withdrawn, either to be returned to the aeration tank as "activated sludge" or wasted as excess. The activated sludge contains aerobic biological life (such as bacteria and protozoa), and it is returned to the aeration tank in order to give this biological life repeated opportunities to metabolize the organic waste. Other material on which the aerobic bacteria and other biological life can feed, including synthesized cell material and soluble organic matter, is also returned from the settling and clarifying tank to the aeration tank as part of the activated sludge.